During production from a cased wellbore, it is often desired to drill lateral drainholes through the casing or open surface of the well to permit further communication between the wellbore and the formation. In the past, such drainholes have been created by lowering a flexible drill shaft and drill bit into the well. The flexible drill shaft is mounted within an elbowed portion which deflects the flexible drill shaft and drill bit to a given (usually 90 degree) angle with respect to the wellbore. The drill shaft must be flexible to accommodate the desired bend, while maintaining sufficient stiffness to enable the flexible shaft to effectively drill the lateral drainhole.
One means for providing flexibility to the shaft is to incorporate a spring within the flexible drill shaft; however, such attempts have been unsuccessful from a practical perspective. For example, U.S. Pat. Nos. 3,838,736 and 4,051,908 to Driver describe flexible drill shafts in which a coil spring is included within the shaft, permitting redirection of the drill as deflected by an elbowed tubing string. In general, such systems are limited in utility with respect to the amount of torque that can be applied to the drill and are highly susceptible to spring failure.
Further, U.S. Pat. No. 6,220,372 to Cherry describes an apparatus for drilling lateral drainholes using a deflection assembly and a dual-spring flexible drill shaft. The deflection assembly includes an elbow and three bearings at the 0.45, and 90 degree positions within the assembly to reduce frictional wear within the system. The deflection assembly is attached to the tubing string and is lowered into the wellbore to a desired height, and the dual spring flexible shaft is then inserted within the tubing string and is deflected by the deflection assembly through a 90 degree angle. The drill shaft includes two counter-coiled springs telescopically engaged with one another such that the application of torque to the springs will tighten the outer spring and expand the inner spring, thereby stiffening the shaft due to the counteracting radial forces between the springs. This type of shaft, although an improvement over the single spring shaft, remains prone to bending stress, metal fatigue, and friction between adjacent coils of each spring, providing limited ability to effectively penetrate either well casing or wellbore. Moreover, advancement of the shaft through a deflection assembly causes the springs to stiffen prematurely, reducing the flexibility of the shaft during deflection.
It is, therefore, desirable to provide an improved drilling system for use in drilling lateral drainholes within a wellbore.